Watch fortress <redacted> is made up entirely of Primaris space marines. The warriors that founded the fortress were originally created by the Alpha Legion, who had succeeded in replicating the process to create Primaris marines for the geneseed of all 18 of the original legions, but only in female subjects. Their mistake was performing these experiments on captured, loyal imperial citizens who once ascended had no compulsion to serve the Alpha Legions interests.
In short order the newly created marines effected an escape from the Alpha Legion, bringing with them the technology to create more of the kind. It took a lot longer to gain the trust of the Imperium, but once their loyalty was proven they found themselves in the service of the Ordo Xenos.
The warriors of the fortress of loyal legion heritage wear the shoulder pads of current space marine chapters that best reflect their individual approach to waging war. Those with heretic legion heritage do not display any heraldry and take up the role of black shields within the watch fortress. Only geneseed of the loyal legions are now used in the creation of new marines, making black shields extremely rare.